Ultrasonic Testing (or UT for short) is one form of nondestructive evaluation technique that uses high frequency sound energy to conduct examinations and make measurements of various materials. Ultrasonic inspection can be used for flaw detection/evaluation, dimensional measurements, material characterization, and other property determinations. In the evaluation of formation rocks, it is often desired to measure properties of formation rock specimens under various pressure and temperature conditions, sometimes under fluid flow conditions.
A typical UT inspection system consists of several functional units, such as the pulser/receiver, transducer, and display devices. A pulser/receiver is an electronic device that can produce high voltage electrical pulses. Driven by the pulser, the transducer generates high frequency ultrasonic energy. The sound energy is introduced and propagates through the materials in the form of sound waves. When the wave path encounters a discontinuity (such as a crack), part of the wave energy will be reflected back from the flaw surface. The reflected wave signal is transformed into an electrical signal by the transducer and may be visually displayed, such as on a screen. The reflected signal strength may be displayed versus the time from signal generation to when an echo was received. Signal travel time can be directly related to the distance that the signal travels. From the signal, information about the reflector location, size, orientation, and other features may be determined. With automated systems, detailed images of materials may be produced.
Ultrasonic testing is based on time-varying deformations or vibrations in materials, which is generally referred to as acoustics. All material substances are comprised of atoms, which may be forced into vibrational motion about their equilibrium positions. Many different patterns of vibrational motion exist at the atomic level; however, most are irrelevant to acoustics and ultrasonic testing. Acoustics is focused on particles that contain many atoms that move in unison to produce a mechanical wave. When a material is not stressed in tension or compression beyond its elastic limit, its individual particles perform elastic oscillations. When the particles of a medium are displaced from their equilibrium positions, internal (electrostatic) restoration forces arise. It is these elastic restoring forces between particles, combined with inertia of the particles that leads to the oscillatory motions of the medium.
In solids, sound waves can propagate in four principle modes that are based on the way the particles oscillate. Sound can propagate as longitudinal waves, shear waves, surface waves, and in thin materials as plate waves. Longitudinal and shear waves are the two modes of propagation most widely used in ultrasonic testing.
In longitudinal waves, the oscillations occur in the longitudinal direction or the direction of wave propagation. Since compressional and dilational forces are active in these waves, they are also called pressure or compressional waves. They are also sometimes called density waves because their particle density fluctuates as they move. Compression waves can be generated in liquids, as well as solids, because the energy travels through the atomic structure by a series of compressions and expansion (rarefaction) movements.
In the transverse or shear wave, the particles oscillate at a right angle or transverse to the direction of propagation. Shear waves require an acoustically solid material for effective propagation, and therefore, are not effectively propagated in materials such as liquids or gasses. Shear waves are relatively weak when compared to longitudinal waves. In fact, shear waves are sometimes generated in materials using some of the energy from longitudinal waves.
As mentioned previously, longitudinal and transverse (shear) waves are most often used in ultrasonic inspection. The conversion of electrical pulses to mechanical vibrations and the conversion of returned mechanical vibrations back into electrical energy is the basis for ultrasonic testing. The active element is the heart of the transducer as it converts the electrical energy to acoustic energy, and vice versa. The active element is basically a piece of polarized material (i.e. some parts of the molecule are positively charged, while other parts of the molecule are negatively charged) with electrodes attached to two of its opposite faces. When an electric field is applied across the material, the polarized molecules will align themselves with the electric field, resulting in induced dipoles within the molecular or crystal structure of the material. This alignment of molecules causes the material to change dimensions. This phenomenon is known as electrostriction. In addition, a permanently-polarized material such as quartz (SiO2) or barium titanate (BaTiO3) will produce an electric field when the material changes dimensions as a result of an imposed mechanical force. This phenomenon is known as the piezoelectric effect.
The active element of most acoustic transducers used today is a piezoelectric ceramic, which can be cut in various ways to produce different wave modes. In particular, p-wave piezoelectric ceramic elements generate primarily longitudinal waves, whereas s-wave piezoelectric ceramic generate primarily transverse (shear) waves.
Preceding the advent of piezoelectric ceramics in the early 1950's, piezoelectric crystals made from quartz crystals and magnetostrictive materials were primarily used. When piezoelectric ceramics were introduced, they soon became the dominant material for transducers due to their good piezoelectric properties and their ease of manufacture into a variety of shapes and sizes. Additionally, they operate at low voltage and are usable up to about 300° C. The first piezoceramic in general use was barium titanate, and that was followed during the 1960's by lead zirconate titanate compositions, which are now the most commonly employed ceramic for making transducers. New materials such as piezo-polymers and composites are also being used in some applications.
The thickness of the active element is determined by the desired frequency of the transducer. A thin wafer element vibrates with a wavelength that is twice its thickness. Therefore, piezoelectric crystals are typically cut to a thickness that is ½ the desired radiated wavelength. The higher the frequency of the transducer, the thinner the active element. The primary reason that high frequency contact transducers are not produced is because the element is very thin and too fragile.
Conventional systems for ultrasonic inspection however suffer from a variety of limitations and disadvantages. Many conventional ultrasonic evaluation devices require the surface material of a specimen to be accessible to transmit ultrasound through the specimen. Such devices are problematic for evaluating rock specimens under pressure for a variety of reasons. For one, conventional devices often employ a coupling medium to promote the transfer of sound energy into the test specimen. This coupling medium is often a conductive gel or glue. Unfortunately, this coupling medium breaks down under certain high pressure/temperature conditions resulting in the coupling medium being limited to a narrow range of conditions. In some cases, the coupling medium can only withstand a few pressure cycles before failing, thus substantially complicating their use.
Where rock specimens are tested in a fluid-filled pressure chamber, the crystal or transducer element is often exposed to the pressurized liquid. Exposure of the crystal to this pressurized fluid can adversely affect measurements. Additionally, corrosive liquids can attack the crystals or transducer elements that are mounted on rock specimens. Where crystals are bonded directly on the specimen, accessing a rock specimen or the crystal itself for maintenance, adjustment, or replacement is difficult and cumbersome.
Some conventional ultrasonic transducers employ transducer elements mounted on a platen which then indirectly interfaces with a rock specimen. Some of these conventional devices use springs, rubber, or other flexible materials to press crystals against the surface of the steel platen. These devices are often overly complicated and frustrate access to transducer element. In some cases, crystals are molded in a resin platen. Again, this arrangement makes access and maintenance of the crystals exceedingly difficult.
Conventional ultrasonic measuring devices using the platen arrangement often have multiple metal interfaces between the transducer element and the rock specimen. These multiple metal interfaces introduce additional measurement errors. Moreover, such devices usually result in excessive distance between the transducer element and the rock specimen. Some conventional ultrasonic measuring devices are arranged such that fluid flow is transverse to the axis of ultrasonic measurement, which further complicates ultrasonic measurements and analysis.
Other conventional devices only have one type of transducer element, a p-wave crystal or an s-wave crystal, thus limiting the amount of useful information that may be obtained by such devices. To circumvent this problem, some conventional devices utilize a stacked disk arrangement, for example a p-crystal as a full disk, immediately on top of an S-full-disk. This arrangement, while allowing for measurement of both longitudinal and shear waves suffers from measurement inaccuracies and errors caused by the inducement of vibration in the one of the crystals by the other crystal. Thus, the stacked configuration allows both types of waves to be measured but results in less accurate measurements that are degraded in resolution. Additionally, in the stacked configuration, p- and s-waves cannot be transmitted at the same time further limiting their usefulness.
Accordingly, there is a need in the art for improved ultrasonic measurement systems and methods that address one or more disadvantages of the prior art.